The orthothecids are one of the two
hyolith orders.[2]
Marek diagnoses the order thus: Conchs with a flat or concave ventral surface — opercula with large, flat cardinal processes but without clavicles – tightly sigmoidal, sediment-filled
intestine – helens absent.
Sometimes the Circothecidae and Tetrathecidae are split out into a separate order 'Circothecida', which is defined by the bottom surface not being flat, the cardinal processes being pronounced, and a circular rim sometimes showing hints of differentiation into clavicles.[3]
^Malinky, J. M. (2009). "Permian Hyolithida from Australia: The Last of the Hyoliths?". Journal of Paleontology. 83: 147–152.
doi:
10.1666/08-094R.1.
S2CID85924056.
^Malinky, J.M.; Skovsted, C.B (2004). "Hyoliths and small shelly fossils from the Lower Cambrian of North−East Greenland". Acta Palaeontologica Polonica. 49 (4): 551–578.
^Kouchinsky, A. V. (2000). "Skeletal microstructures of hyoliths from the Early Cambrian of Siberia". Alcheringa. 24 (2): 65–81.
doi:
10.1080/03115510008619525.
S2CID140660142.